In the loop join algorithm, an outer loop is formed that is composed of a few entries that are to be selected. Then, for each entry in the outer loop, a look-up is performed for matching entries, in the inner loop.
In the merge join algorithm, both tables are accessed in the same order. If there's a sorted index on the matching column, on both tables, then no sorting is needed. All we have to do is read the rows in the order presented by the index. The reason it's called a merge join is that the algorithm, in detail, looks much like the algorithm for merging two (sorted) data streams together.
Let's say we got two tables, ORDERS and ORDER_ITEMS. Let's say we have sorted indexes on ORDER_NUMBER on both tables. Naturally, the index on ORDERS can forbid duplicates, while the index on ORDER_ITEMS has to permit duplicates.
Now, in this case, which algorithm is faster? It depends.
Let's say we want to look up a single order. This happens in OLTP systems a lot. The loop join is probably faster. The outer loop will find a single order number, and that means the inner loop will have to probe the index on ORDER_ITEMS just once. This is true even if we have to scan the order table, based on CUSTOMER_ID and ORDER_DATE.
Now let's say we want a report for all the reports, with details for April. The merge join is probably faster. With hundreds of orders to process, walking the index on ORDER_ITEMS once beats the heck out of doing hundreds of probes.
Fully Hinting Comment by Jonathan Lewis on USENET.
Consider, for example: SELECT /*+ index(t1 t1_abc) index(t2 t2_abc) */ COUNT(*) FROM t1, t2 WHERE t1.col1 = t2.col1;
For weeks, this may give you the plan:
NESTED LOOP table access by rowid t1 index range scan t1_abc table access by rowid t2 index range scan t2_abc
Then, because of changes in statistics, or init.ora parameters, or nullity of a column, or a few other situations that may have slipped my mind at the moment, this might change to HASH JOIN table access by rowid t2 index range scan t2_abc table access by rowid t1 index range scan t1_abc
Your hints are still obeyed, the plan has changed. On the other hand, if you had specified
Then I think you could be fairly confident that there was no way that Oracle could obey the hints whilst changing the access path.
Optimizer Approaches
ALL_ROWS
The ALL_ROWS hint explicitly chooses the cost-based approach to optimize a statement block with a goal of best throughput (that is, minimum total resource consumption). /*+ ALL_ROWS */
conn / as sysdba
set linesize 121 col name format a30 col value format a30
SELECT name, value FROM gv$parameter WHERE name LIKE '%optimizer%';
ALTER SYSTEM SET optimizer_mode=RULE SCOPE=MEMORY;
set autotrace traceonly explain
SELECT table_name FROM dba_tables WHERE owner = 'SYS' AND table_name LIKE '%$' ORDER BY 1;
SELECT /*+ ALL_ROWS */ table_name FROM dba_tables WHERE owner = 'SYS' AND table_name LIKE '%$' ORDER BY 1;
ALTER SYSTEM SET optimizer_mode=ALL_ROWS SCOPE=MEMORY;
FIRST_ROWS(n)
The FIRST_ROWS hint explicitly chooses the cost-based approach to optimize a statement block with a goal of best response time (minimum resource usage to return first row).
This hint causes the optimizer to make these choices:
If an index scan is available, the optimizer may choose it over a full table scan.
If an index scan is available, the optimizer may choose a nested loops join over a sort-merge join whenever the associated table is the potential inner table of the nested loops.
If an index scan is made available by an ORDER BY clause, the optimizer may choose it to avoid a sort operation.
The optimizer ignores this hint in DELETE and UPDATE statement blocks and in SELECT statement blocks that contain any of the following: UNION, INTERSECT, MINUS, UNION ALL, GROUP BY, FOR UPDATE, aggregating function and the DISTINCT operator.
/*+ FIRST_ROWS() */
set autotrace trace exp
SELECT table_name FROM dba_tables WHERE owner = 'SYS' AND table_name LIKE '%$' ORDER BY 1;
SELECT /*+ FIRST_ROWS(10) */ table_name FROM dba_tables WHERE owner = 'SYS' AND table_name LIKE '%$' ORDER BY 1;
RULE
Disables the use of the optimizer. This hint is not supported and should not be used. /*+ RULE */
set autotrace trace exp SELECT table_name FROM dba_tables WHERE owner = 'SYS' AND table_name LIKE '%$' ORDER BY 1;
SELECT /*+ RULE */ table_name FROM dba_tables WHERE owner = 'SYS' AND table_name LIKE '%$' ORDER BY 1;
General
APPEND
instructs the optimizer to use direct-path INSERT if your database is running in serial mode. Your database is in serial mode if you are not using Enterprise Edition. Conventional INSERT is the default in serial mode, and direct-path INSERT is the default in parallel mode.
In direct-path INSERT, data is appended to the end of the table, rather than using existing space currently allocated to the table. As a result, direct-path INSERT can be considerably faster than conventional INSERT.
When you use the APPEND hint for INSERT, data is simply appended to a table above the HWM which has the effect of not creating UNDO. Existing free space in blocks is not used. /*+ APPEND */
-- demo that with APPEND indexes are bulk loaded at the end of -- the insert.
CREATE TABLE t AS SELECT * FROM all_objects;
CREATE INDEX ix1_t ON t(owner, object_name, object_type);
CREATE INDEX ix2_t ON t(object_id);
INSERT INTO t SELECT * FROM all_objects;
SELECT used_ublk FROM gv$transaction;
ROLLBACK;
INSERT /*+ APPEND */ INTO t SELECT * FROM all_objects;
SELECT used_ublk FROM gv$transaction;
CACHE
Instructs the optimizer to place the blocks retrieved for the table at the most recently used end of the LRU list in the buffer cache when a full table scan is performed. This hint is useful for small lookup tables. /*+ CACHE([@queryblock] ) */
SELECT /*+ FULL (hr_emp) CACHE(hr_emp) */ last_name FROM employees hr_emp;
CURSOR_SHARING_EXACT
Oracle can replace literals in SQL statements with bind variables, when it is safe to do so. This replacement is controlled with the CURSOR_SHARING initialization parameter. The CURSOR_SHARING_EXACT hint instructs the optimizer to switch this behavior off. In other words, Oracle executes the SQL statement without any attempt to replace literals with bind variables.
/*+ CURSOR_SHARING_EXACT */
ALTER SYSTEM SET cursor_sharing='SIMILAR' SCOPE=BOTH;
ALTER SYSTEM FLUSH SHARED_POOL; ALTER SYSTEM FLUSH SHARED_POOL;
-- as the client run two similar SQL statements SELECT latitude FROM uwclass.servers WHERE srvr_id = 1; SELECT latitude FROM uwclass.servers WHERE srvr_id = 2; SELECT latitude FROM uwclass.servers WHERE srvr_id = 3;
-- as SYS look in the shared pool set linesize 121 col sql_text format a50
SELECT address, child_address, sql_text, sql_id FROM gv$sql WHERE sql_fulltext LIKE '%uwclass%';
SELECT /*+ CURSOR_SHARING_EXACT */ latitude FROM uwclass.servers WHERE srvr_id = 3;
SELECT address, child_address, sql_text, sql_id FROM gv$sql WHERE sql_fulltext LIKE '%uwclass%';
DRIVING_SITE
Forces query execution to be done at a user selected site rather than at a site selected by the database. This hint is useful if you are using distributed query optimization. /*+ DRIVING_SITE([@queryblock] ) */
SELECT /*+ DRIVING_SITE(departments) */ * FROM employees, departments@rsite WHERE employees.department_id = departments.department_id;
DYNAMIC_SAMPLING
The DYNAMIC_SAMPLING hint instructs the optimizer how to control dynamic sampling to improve server performance by determining more accurate predicate selectivity and statistics for tables and indexes.
You can set the value of DYNAMIC_SAMPLING to a value from 0 to 10. The higher the level, the more effort the compiler puts into dynamic sampling and the more broadly it is applied. Sampling defaults to cursor level unless you specify tablespec. The integer value is 0 to 10, indicating the degree of sampling.
Force dynamic sampling of tables where statistics do not exist such as Global Temporary Tables.
If the table is aliased the alias name, not the table name must be used /*+ DYNAMIC_SAMPLING([@queryblock] [] ) */
CREATE TABLE ds AS SELECT * FROM all_objects;
CREATE INDEX ds_objtype ON ds(object_type);
SELECT object_type, COUNT(*) FROM ds GROUP BY object_type; set autotrace trace exp
SELECT object_name FROM ds WHERE object_type = 'JAVA CLASS';
SELECT /*+ DYNAMIC_SAMPLING(ds 0) */ object_name FROM ds WHERE object_type = 'JAVA CLASS';
SELECT /*+ DYNAMIC_SAMPLING(ds 4) */ object_name FROM ds WHERE object_type = 'JAVA CLASS';
SELECT /*+ DYNAMIC_SAMPLING(ds 9) */ object_name FROM ds WHERE object_type = 'JAVA CLASS';
MODEL_MIN_ANALYSIS
Instructs the optimizer to omit some compile-time optimizations of spreadsheet rules�primarily detailed dependency graph analysis. Other spreadsheet optimizations, such as creating filters to selectively populate spreadsheet access structures and limited rule pruning, are still used by the optimizer.
This hint reduces compilation time because spreadsheet analysis can be lengthy if the number of spreadsheet rules is more than several hundreds. /*+ MODEL_MIN_ANALYSIS */
TBD
NOAPPEND
Instructs the optimizer to use conventional INSERT by disabling parallel mode for the duration of the INSERT statement. Conventional INSERT is the default in serial mode, and direct-path INSERT is the default in parallel mode. /*+ NOAPPEND */
INSERT /*+ NOAPPEND */ INTO t SELECT * FROM all_objects;
NO_CACHE
Specifies that the blocks retrieved for this table are placed at the least recently used end of the LRU list in the buffer cache when a full table scan is performed. This is the normal behavior of blocks in the buffer cache. /*+ NOCACHE([@queryblock] ]) */
SELECT /*+ FULL(hr_emp) NOCACHE(hr_emp) */ last_name FROM employees hr_emp;
NO_PUSH_PRED
Instructs the optimizer not to push a join predicate into the view. /*+ NO_PUSH_PRED( | ]) */
SELECT /*+ NO_MERGE(v) NO_PUSH_PRED(v) */ * FROM employees e, ( SELECT manager_id FROM employees) v WHERE e.manager_id = v.manager_id(+) AND e.employee_id = 100;
NO_PUSH_SUBQ
Instructs the optimizer to evaluate nonmerged subqueries as the last step in the execution plan. Doing so can improve performance if the subquery is relatively expensive or does not reduce the number of rows significantly. /*+ NO_PUSH_SUBQ() */
TBD
NO_PX_JOIN_FILTER
Prevents the optimizer from using parallel join bitmap filtering. /*+ NO_PX_JOIN_FILTER() */
TBD
NO_XML_QUERY_REWRITE
Instructs the optimizer to prohibit the rewriting of XPath expressions in SQL statements. /*+ NO_XML_QUERY_REWRITE */
SELECT /*+NO_XML_QUERY_REWRITE*/ XMLQUERY('') FROM dual;
PUSH_PRED
Instructs the optimizer to push a join predicate into the view. /*+ PUSH_PRED( | ]) */
conn hr/hr set autotrace trace exp SELECT * FROM employees e, ( SELECT manager_id FROM employees) v WHERE e.manager_id = v.manager_id(+) AND e.employee_id = 100; SELECT /*+ NO_MERGE(v) PUSH_PRED(v) */ * FROM employees e, ( SELECT manager_id FROM employees) v WHERE e.manager_id = v.manager_id(+) AND e.employee_id = 100;
PUSH_SUBQ
Instructs the optimizer to evaluate nonmerged subqueries at the earliest possible step in the execution plan. Generally, subqueries that are not merged are executed as the last step in the execution plan. If the subquery is relatively inexpensive and reduces the number of rows significantly, then evaluating the subquery earlier can improve performance.
This hint has no effect if the subquery is applied to a remote table or one that is joined using a merge join. /*+ PUSH_SUBQ() */
TBD
PX_JOIN_FILTER
Forces the optimizer to use parallel join bitmap filtering. /*+ PX_JOIN_FILTER() */
TBD
QB_NAME
Use the QB_NAME hint to define a name for a query block. This name can then be used in a hint in the outer query or even in a hint in an inline view to affect query execution on the tables appearing in the named query block.
If two or more query blocks have the same name, or if the same query block is hinted twice with different names, then the optimizer ignores all the names and the hints referencing that query block. Query blocks that are not named using this hint have unique system-generated names. These names can be displayed in the plan table and can also be used in hints within the query block, or in query block hints. /*+ qb_name() */
SELECT /*+ QB_NAME(qb) FULL(@qb e) */ employee_id, last_name FROM employees e WHERE last_name = 'Smith';
Access Method Hints
Each following hint described in this section suggests an access method for a table.
FULL
Explicitly chooses a full table scan for the specified table /*+ FULL() */
conn uwclass/uwclass
set autotrace traceonly explain
SELECT latitude FROM servers WHERE srvr_id = 1;
SELECT /*+ FULL(servers) */ latitude FROM servers WHERE srvr_id = 1;
INDEX
Explicitly chooses an index scan for the specified table. You can use the INDEX hint for domain, B*-tree, and bitmap indexes. However, Oracle recommends using INDEX_COMBINE rather than INDEX for bitmap indexes because it is a more versatile hint /*+ INDEX([@queryblock] ) */
conn oe/oe CREATE INDEX ix_customers_gender ON customers(gender);
set autotrace traceonly explain
SELECT * FROM customers WHERE gender = 'M';
SELECT /*+ INDEX(customers ix_customers_gender) */ * FROM customers WHERE gender = 'M';
INDEX_ASC
Explicitly chooses an index scan for the specified table. If the statement uses an index range scan, Oracle scans the index entries in ascending order of their indexed values /*+ INDEX_ASC([@queryblock] ) */
conn oe/oe
CREATE INDEX ix_customers_gender ON customers(gender);
set autotrace traceonly explain
SELECT * FROM customers WHERE gender = 'M';
SELECT /*+ INDEX_ASC(customers ix_customers_gender) */ * FROM customers WHERE gender = 'M';
INDEX_COMBINE
Explicitly chooses a bitmap access path for the table. If no indexes are given as arguments for the INDEX_COMBINE hint, the optimizer uses whatever Boolean combination of bitmap indexes has the best cost estimate for the table. If certain indexes are given as arguments, the optimizer tries to use some Boolean combination of those particular bitmap indexes. /*+ INDEX_DESC([@queryblock] ) */
conn hr/hr
set autotrace traceonly explain
SELECT * FROM employees e WHERE (manager_id = 108 OR department_id = 110);
SELECT /*+ INDEX_COMBINE(e emp_manager_ix emp_department_ix) */ * FROM employees e WHERE (manager_id = 108 OR department_id = 110);
INDEX_DESC
Explicitly chooses an index scan for the specified table. If the statement uses an index range scan, Oracle scans the index entries in descending order of their indexed values. /*+ INDEX_DESC([@queryblock] ) */
conn oe/oe
set autotrace traceonly explain
SELECT * FROM customers WHERE gender = 'M';
SELECT /*+ INDEX_DESC(customers ix_customers_gender) */ * FROM customers WHERE gender = 'M';
INDEX_FFS
Causes a fast full index scan rather than a full table scan /*+ INDEX_FFS([@queryblock] ) */
conn oe/oe
set autotrace traceonly explain
SELECT first_name FROM employees e;
SELECT /*+ INDEX_FFS(e emp_name_ix) */ first_name FROM employees e;
INDEX_JOIN
Explicitly instructs the optimizer to use an index join as an access path. For the hint to have a positive effect, a sufficiently small number of indexes must exist that contain all the columns required to resolve the query. /*+ INDEX_JOIN([@queryblock] ) */
conn oe/oe
set autotrace traceonly explain
SELECT department_id FROM employees e WHERE manager_id < 110 AND department_id < 50;
--------------------------------------------------------------------- | Id | Operation | Name | Cost(%CPU)| --------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 2 (0) | |* 1 | TABLE ACCESS BY INDEX ROWID | EMPLOYEES | 2 (0) | |* 2 | INDEX RANGE SCAN | EMP_DEPARTMENT_IX | 1 (0) | ---------------------------------------------------------------------
SELECT /*+ INDEX_JOIN(e emp_manager_ix emp_department_ix) */ department_id FROM employees e WHERE manager_id < 110 AND department_id < 50;
Instructs the optimizer to perform an index skip scan for the specified table. If the statement uses an index range scan, then Oracle scans the index entries in ascending order of their indexed values. In a partitioned index, the results are in ascending order within each partition. /*+ INDEX_SS([@queryblock] ) */
conn oe/oe
set autotrace traceonly explain
SELECT last_name FROM employees e WHERE first_name = 'Steven';
SELECT /*+ INDEX_SS(e emp_name_ix) */ last_name FROM employees e WHERE first_name = 'Steven';
INDEX_SS_ASC
Instructs the optimizer to perform an index skip scan for the specified table. If the statement uses an index range scan, then Oracle Database scans the index entries in ascending order of their indexed values. In a partitioned index, the results are in ascending order within each partition. Each parameter serves the same purpose as in "INDEX Hint".
The default behavior for a range scan is to scan index entries in ascending order of their indexed values, or in descending order for a descending index. This hint does not change the default order of the index, and therefore does not specify anything more than the INDEX_SS hint. However, you can use the INDEX_SS_ASC hint to specify ascending range scans explicitly should the default behavior change. /*+ INDEX_SS_ASC([@queryblock] ) */
TBD
INDEX_SS_DESC
Instructs the optimizer to perform an index skip scan for the specified table. If the statement uses an index range scan and the index is ascending, then Oracle scans the index entries in descending order of their indexed values. In a partitioned index, the results are in descending order within each partition. For a descending index, this hint effectively cancels out the descending order, resulting in a scan of the index entries in ascending order.
/*+ INDEX_SS_DESC([@queryblock] ) */
conn oe/oe
set autotrace traceonly explain
SELECT last_name FROM employees e WHERE first_name = 'Steven';
SELECT /*+ INDEX_SS_DESC(e emp_name_ix) */ last_name FROM employees e WHERE first_name = 'Steven';
NO_INDEX
Explicitly disallows a set of indexes for the specified table. The NO_INDEX hint applies to function-based, B*-tree, bitmap, cluster, or domain indexes. /*+ NO_INDEX([@queryblock] ) */
conn uwclass/uwclass
set autotrace traceonly explain
SELECT latitude FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;
SELECT /*+ NO_INDEX(i ix_serv_inst) */latitude FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;
NO_INDEX_FFS
Instructs the optimizer to exclude a fast full index scan of the specified indexes.
/*+ NO_INDEX_FFS([@queryblock] ) */
SELECT /*+ NO_INDEX_FFS(i pk_serv_inst) NO_INDEX_FFS(i ix_serv_inst) */ latitude FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;
NO_INDEX_SS
Instructs the optimizer to exclude a skip scan of the specified indexes.
/*+ NO_INDEX_SS([@queryblock] ) */
SELECT /*+ NO_INDEX_SS(i ix_serv_inst) */ latitude FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;
Cluster Only Access Method Hints
Each following hint can only be used with clusters.
CLUSTER
explicitly chooses a cluster scan to access the specified table. Only applies to clusters. /*+ CLUSTER([@queryblock] ) */
TBD
HASH
Explicitly chooses a hash scan to access the specified table. Only applies to clusters. /*+ HASH() */
TBD
Join Order
The hints in this section suggest join orders:
LEADING
Instructs the optimizer to use the specified set of tables as the prefix in the execution plan. /*+ LEADING([@queryblock] ) */
conn hr/hr
set autotrace traceonly explain
SELECT * FROM employees e, departments d, job_history j WHERE e.department_id = d.department_id AND e.hire_date = j.start_date;
SELECT /*+ LEADING(e j) */ * FROM employees e, departments d, job_history j WHERE e.department_id = d.department_id AND e.hire_date = j.start_date;
ORDERED
Causes Oracle to only join tables in the order in which they appear in the FROM clause. /*+ ORDERED */
conn oe/oe set autotrace traceonly explain SELECT o.order_id, c.customer_id, l.unit_price * l.quantity FROM customers c, order_items l, orders o WHERE c.cust_last_name = 'Mastroianni' AND o.customer_id = c.customer_id AND o.order_id = l.order_id;
SELECT /*+ORDERED */ o.order_id, c.customer_id, l.unit_price * l.quantity FROM customers c, order_items l, orders o WHERE c.cust_last_name = 'Mastroianni' AND o.customer_id = c.customer_id AND o.order_id = l.order_id;
Join Operation
Each hint described in this section suggests a join operation for a table.
NO_USE_HASH (table1 table2)
Instructs the optimizer to exclude hash joins when joining each specified table to another row source using the specified table as the inner table. /*+ NO_USE_HASH([@queryblock] ) */
TBD
NO_USE_MERGE (table1 table2)
Instructs the optimizer to exclude sort-merge joins when joining each specified table to another row source using the specified table as the inner table.
/*+ NO_USE_MERGE([@queryblock] ) */
conn oe/oe
set autotrace traceonly explain
SELECT * FROM employees e, departments d WHERE e.department_id = d.department_id;
SELECT /*+ NO_USE_MERGE(e d) */ * FROM employees e, departments d WHERE e.department_id = d.department_id;
NO_USE_NL (table1 table2)
Instructs the optimizer to exclude nested loops joins when joining each specified table to another row source using the specified table as the inner table.
/*+ NO_USE_NL([@queryblock] ) */
conn oe/oe
set autotrace traceonly explain
SELECT * FROM orders h, order_items l WHERE l.order_id = h.order_id AND l.order_id > 3500;
SELECT /*+ NO_USE_NL(l h) */ * FROM orders h, order_items l WHERE l.order_id = h.order_id AND l.order_id > 3500;
USE_HASH (table1 table2)
Causes Oracle to join each specified table with another row source with a hash join. /*+ USE_HASH([@queryblock] ) */
conn uwclass/uwclass
set autotrace traceonly explain
SELECT DISTINCT s.srvr_id FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;
SELECT /*+ USE_HASH (s i) */ DISTINCT s.srvr_id FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;
USE_MERGE (table1 table2)
Causes Oracle to join each specified table with another row source with a sort-merge join. /*+ USE_MERGE([@queryblock] ) */
conn uwclass/uwclass
set autotrace traceonly explain
SELECT DISTINCT s.srvr_id FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;
SELECT /*+ USE_MERGE (s i) */ DISTINCT s.srvr_id FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;
USE_NL (table1 table2)
Causes Oracle to join each specified table to another row source with a nested loops join using the specified table as the inner table. /*+ USE_NL([@queryblock] ) */
conn uwclass/uwclass
set autotrace traceonly explain
SELECT DISTINCT s.srvr_id FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;
SELECT /*+ USE_NL (s i) */ DISTINCT s.srvr_id FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;
USE_NL_WITH_INDEX
Instructs the optimizer to join the specified table to another row source with a nested loops join using the specified table as the inner table. /*+ USE_NL_WITH_INDEX([@queryblock] ) */
SELECT /*+ USE_NL_WITH_INDEX(l item_product_ix) */ * FROM orders h, order_items l WHERE l.order_id = h.order_id AND l.order_id > 3500;
Parallel Execution
The hints described in this section determine how statements are parallelized or not parallelized when using parallel execution.
PARALLEL
Specify the desired number of concurrent servers that can be used for a parallel operation. The hint applies to the INSERT, UPDATE, and DELETE portions of a statement as well as to the table scan portion. If any parallel restrictions are violated, the hint is ignored. /*+ PARALLEL([@queryblock] ) */
-- overrides table definition and uses init parmameter SELECT /*+ FULL(hr_emp) PARALLEL(hr_emp, DEFAULT) */ last_name FROM employees hr_emp;
NO_PARALLEL
Overrides a PARALLEL specification in the table clause. In general, hints take precedence over table clauses. /*+ NO_PARALLEL([@queryblock] ) */
SELECT /*+ NO_PARALLEL(hr_emp) */ last_name FROM employees hr_emp;
PQ_DISTRIBUTE
Improve parallel join operation performance. Do this by specifying how rows of joined tables should be distributed among producer and consumer query servers. Using this hint overrides decisions the optimizer would normally make.
Outer_distribution is the distribution for the outer table. Inner_distribution is the distribution for the inner table. /*+ PQ_DISTRIBUTE([@queryblock] ) */
TBD
PARALLEL_INDEX
Specify the desired number of concurrent servers that can be used to parallelize index range scans for partitioned indexes. /*+ PARALLEL_INDEX([@queryblock] ) */
TBD
NO_PARALLEL_INDEX
Override a PARALLEL attribute setting on an index. In this way you can avoid a parallel index scan operation. /*+ NO_PARALLEL_INDEX([@queryblock] ) */
TBD
Query Transformation
FACT
In the context of the star transformation. It instructs the optimizer that the table specified in table specification should be considered as a fact table.
/*+ FACT([@queryblock] ) */
TBD
NO_EXPAND
Prevents the cost-based optimizer from considering OR-expansion for queries having OR conditions or INLISTS in the WHERE clause. Normally, the optimizer would consider using OR expansion and use this method if it decides the cost is lower than not using it. /*+ FACT();
conn oe/oe
set autotrace traceonly explain
SELECT * FROM employees e, departments d WHERE e.manager_id = 108 OR d.department_id = 110;
SELECT /*+ NO_EXPAND */ * FROM employees e, departments d WHERE e.manager_id = 108 OR d.department_id = 110;
NO_FACT
Used in the context of the star transformation. It instruct the optimizer that the queried table should not be considered as a fact table.
/*+ NO_FACT([@queryblock] ) */
TBD
NO_QUERY_TRANSFORMATION
Instructs the optimizer to skip all query transformations, including but not limited to OR-expansion, view merging, subquery unnesting, star transformation, and materialized view rewrite.
/*+ NO_QUERY_TRANSFORMATION) */
conn oe/oe
set autotrace traceonly explain
SELECT employee_id, last_name FROM ( SELECT * FROM employees e) v WHERE v.last_name = 'Smith';
SELECT /*+ NO_QUERY_TRANSFORMATION */ employee_id, last_name FROM ( SELECT * FROM employees e) v WHERE v.last_name = 'Smith';
NO_REWRITE
Use on any query block of a request. This hint disables query rewrite for the query block, overriding the setting of the parameter QUERY_REWRITE_ENABLED. /*+ NO_REWRITE() */
SELECT /*+ NO_REWRITE */ SUM(s.amount_sold) AS dollars FROM sales s, times t WHERE s.time_id = t.time_id GROUP BY t.calendar_month_desc;
NO_STAR_TRANSFORMATION
Instructs the optimizer not to perform star query transformation. /*+ NO_STAR_TRANSFORMATION() */
TBD
NO_UNNEST
Turns off unnesting of subqueries /*+ NO_UNNEST() */
set autotrace on
SELECT srvr_id FROM servers WHERE srvr_id IN ( SELECT /*+ unnest */ srvr_id FROM serv_inst);
Predicate Information (identified by operation id): ---------------------------------------------------
4 - access("SRVR_ID"="SRVR_ID")
SELECT srvr_id FROM servers WHERE srvr_id IN ( SELECT /*+ no_unnest */ srvr_id FROM serv_inst);
-------------------------------------------------------------- | Id | Operation | Name | Rows | Cost(%CPU) | -------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 72 (0) | |* 1 | INDEX FULL SCAN | PK_SERVERS | 7 | 1 (0) | |* 2 | INDEX RANGE SCAN | IX_SERV_INST | 2 | 1 (0) | --------------------------------------------------------------
Predicate Information (identified by operation id): ---------------------------------------------------
1 - filter( EXISTS (SELECT /*+ NO_UNNEST */ 0 FROM "SERV_INST" "SERV_INST" WHERE "SRVR_ID"=:B1)) 2 - access("SRVR_ID"=:B1)
REWRITE
Use with or without a view list. If you use REWRITE with a view list and the list contains an eligible materialized view, Oracle uses that view regardless of its cost. Oracle does not consider views outside of the list. If you do not specify a view list, Oracle searches for an eligible materialized view and always uses it regardless of its cost. /*+ REWRITE([@queryblock] ) */
TBD
STAR_TRANSFORMATION
Makes the optimizer use the best plan in which the transformation has been used. Without the hint, the optimizer could make a cost-based decision to use the best plan generated without the transformation, instead of the best plan for the transformed query.
Even if the hint is given, there is no guarantee that the transformation will take place. The optimizer will only generate the subqueries if it seems reasonable to do so. If no subqueries are generated, there is no transformed query, and the best plan for the untransformed query will be used regardless of the hint. /*+ STAR_TRANSFORMATION() */
SELECT /*+ STAR_TRANSFORMATION */ * FROM sales s, times t, products p, channels c WHERE s.time_id = t.time_id AND s.prod_id = p.product_id AND s.channel_id = c.channel_id AND p.product_status = 'obsolete';
UNNEST
Instructs the optimizer to unnest and merge the body of the subquery into the body of the query block that contains it, allowing the optimizer to consider them together when evaluating access paths and joins. /*+ UNNEST() */
See NO_UNNEST above
USE_CONCAT
Forces combined OR conditions in the WHERE clause of a query to be transformed into a compound query using the UNION ALL set operator. Normally, this transformation occurs only if the cost of the query using the concatenations is cheaper than the cost without them.
The USE_CONCAT hint turns off inlist processing and OR-expands all disjunctions, including inlists.
conn hr/hr
set autotrace traceonly explain
SELECT * FROM employees e WHERE manager_id = 108 OR department_id = 110;
SELECT /*+ USE_CONCAT */ * FROM employees e WHERE manager_id = 108 OR department_id = 110;
Others
ANTIJOIN
TBD
BITMAP
TBD
BUFFER
TBD
CARDINALITY
SELECT /*+ cardinality(gtt 999) */ * FROM gtt;
HASH_AJ
TBD
INLINE
TBD
MATERIALIZE
TBD
MERGE
Merge a view on a per-query basis
TBD
NO_ACCESS
TBD
NO_BUFFER
TBD
NO_MERGE
Instructs the optimizer not to combine the outer query and any inline view queries into a single query
TBD
NO_MONITORING
Do not update the col_usage$ table with column usage information for this statement
